In recent years, many containers such as PET bottles and metal cans are covered with shrink labels prepared by performing printing on a base film of a heat-shrinkable resin film.
As the shrink label, labels comprising polystyrene type resin predominate since it is superior in low temperature shrinkability. However, since the polystyrene type resin film has inadequate heat resistance, there was a problem that for example, the shrink label may be shrunk and the label may be distorted or broken when a PET bottle topples in being heated in a hot warmer equipped in a convenience store, and the like. Furthermore, the polystyrene type resin film also has a problem that since it has inadequate solvent resistance, it may be shrunk or dissolved by the adhesion of oils when it is used for the packaging of articles containing oils.
On the other hand, efforts to use a polyester type film, which is superior in low temperature resistance, heat resistance and solvent resistance, in place of a polystyrene type resin film as a shrink label are made. But, the polyester type film has a problem that it tends to produce crinkles when it covers a container since it has bad low temperature shrinkability and is shrunk rapidly. Further, perforation for tearing off is often provided on the shrink film so that the shrink label can be easily torn off and removed from the used container in order to recycle containers, but in the polyester type film, there was also a problem that tearing properties along the perforation are poor, and therefore the shrink label cannot be easily torn off and removed from the container.
For this situation, in Patent Document 1, a hard multilayer shrinkable film obtainable by laminating outer surface layers comprising a polyester type resin on an intermediate layer comprising a polystyrene type resin with an adhesive layer comprising an olefin type resin interposed between the outer surface layer and the intermediate layer, is disclosed. Further, in Patent Document 2, a shrink label, which includes a base film obtainable by laminating outer surface layers comprising a polyester type resin comprising a specific monomer on both sides of an intermediate layer comprising a polystyrene type resin, and by laminating the outer surface layers on the intermediate layer without an adhesive layer interposed between the outer surface layer and the intermediate layer, is disclosed. The shrink labels comprising these multilayer films are superior in low temperature shrinkability and tearing properties along the perforation by the virtue of the intermediate layer comprising a polystyrene type resin, and further they are also superior in solvent resistance, low temperature resistance and heat resistance since the intermediate layer is covered with the outer surface layers comprising a polyester type resin. However, if these shrink labels cover the containers actually, there were problems that in the hard multilayer shrinkable film described in Patent Document 1, the intermediate layer may be peeled off from the outer surface layer in covering the container, and in the shrink label described in Patent Document 2, after covering the container with the shrink label, if the films rub against each other or the film is scratched with a fingernail or a body during transportation of products, peeling may occur between an intermediate layer and an outer surface layer.
Additionally, in recent years, when a shrink label formed by performing printing on a base film of the heat-shrinkable resin film covers a plastic container such as a PET bottle, it is seen as a problem that the contents of a container are deteriorated and a storage property becomes worse due to ultraviolet light emitted from solar light or a fluorescent lamp compared with the storage by a metal can. For example, chlorophyll contained in an edible oil and the like is decomposed at a wavelength of 300 nm or shorter and causes quality deterioration. Furthermore, citral used as an aromatic of beverage and the like changes to a component having unfavorable fragrance by light with a wavelength of 300 to 350 nm. Further, amino acids such as tryptophan and thyrosin contained in sake are discolored by light with a wavelength of 320 to 380 nm. For this reasons, in recent years, shrink films having an ultraviolet light blocking property to prevent the deterioration/change of savor and taste and discoloration/fading of the contents of the containers due to ultraviolet light are in practical use.
As such a shrink label, labels comprising polystyrene type resin predominate since it is superior in low temperature shrinkability, and for example, in Patent Document 3, an ultraviolet light absorbent polystyrene type resin heat-shrinkable film, which is a polystyrene type resin film having a three-layer structure, and has a larger amount of an ultraviolet absorber mixed in the intermediate layer than in both outer layers, is disclosed.
However, the heat-shrinkable polystyrene type film having such a constitution had a problem that the film cannot adequately absorb light with a wavelength of 400 nm or shorter if the ultraviolet absorber is not added in a large amount. Further, it had problem that the ultraviolet absorber is bled out and this film becomes economically disadvantageous when the ultraviolet absorber is added in a large amount. Further, the polystyrene type film also has a problem that since it has inadequate oil resistance, it may be shrunk or dissolved by the adhesion of oils when a label of polystyrene type film covers a container of contents containing oils.
On the other hand, efforts to use a heat-shrinkable polyester type film formed by adding an ultraviolet absorber to the polyester type film, which is superior in low temperature resistance, solvent resistance and oil resistance, in place of the polystyrene type resin film as a shrink label are made, and for example, in Patent Document 4, a polyester type resin film, which realizes excellent ultraviolet light blocking properties and heat-shrinkability by a method of adding an ultraviolet absorber to the heat-shrinkable polyester type film, or by a method of adding an ultraviolet absorber to an inner layer formed by laminating polyester type films by two or more layers, is disclosed.
However, since the polyester type films have a high molding temperature, in the case of extrusion molding the polyester type resin, the sublimation and evaporation of the ultraviolet absorber from a die leads to a hostile working environment and causes the contamination of a roll to take off a film extruded from the die to deteriorate productivity. If the molding temperature is lowered to prevent the thermal deterioration of the ultraviolet absorber, the productivity of the film is significantly deteriorated. Furthermore, in the polyester type film, there is also a problem that tearing properties along the perforation are poor, and therefore the shrink label cannot be easily torn off and removed from the container.
Accordingly, a heat-shrinkable multilayer resin film having an excellent ultraviolet light blocking property and an excellent tearing property along the perforation without leading to the hostile working environment and causing reduction in the productivity has be needed.    Patent Document 1: Japanese Kokai Publication Sho-61-41543    Patent Document 2: Japanese Kokai Publication 2002-351332    Patent Document 3: Japanese Kokai Publication 2002-326324    Patent Document 4: Japanese Kokai Publication 2001-323082